15 December 2006

According to a study published in the British Medical Journal1, children with higher IQ scores at age 10 years are more likely to be vegetarians at age 30. (Also read the BBC news article.)

The study included 8170 men and women aged 30 years whose IQs were tested when they were 10 years old. The mean childhood IQ scores of vegetarians were 106.1 and 104.0 for men and women, respectively. In comparison, the mean childhood IQ scores of non-vegetarians were 100.6 and 99.0 for men and women, respectively.

Obviously, the IQ differences between vegetarians and non-vegetarians are not drastic, especially when the overall standard deviation of 15 (mean=100) is taken into account.

The authors intended to test the hypothesis that vegetarianism might be the reason why more intelligent people appear to have a lower risk of coronary heart disease. (The link between intelligence and coronary heart disease risk was demonstrated in earlier studies.)

The authors offer 2 explanations of their results. One is that children with greater intelligence are more likely to become vegetarians and, therefore, are less likely to suffer from coronary heart disease. (The presumed link between vegetarianism and coronary heart disease risk was also demonstrated in earlier studies.)

The 2nd explanation is that there is no casual link between intelligence and being a vegetarian. Intelligent people are more likely to take better care of their health and make smarter choices about other lifestyle preferences that may lower their risk of coronary heart disease.

In my mind, the relatively small IQ differences that were observed between vegetarians and non-vegetarians and the uncertainty about what exactly is a "vegetarian" make it difficult to come up with a clear-cut interpretation of the results. The authors note that 33.6% of the vegetarians in the study group admitted eating fish or chicken and that no difference was found between the IQ scores of those 2 groups. Does that mean a person's IQ is a factor in deciding only whether he or she will eat red meat?

What about vegans (those who eat no animal products)? Are they even smarter?

Well, it turns out that the vegans in the group had a mean childhood IQ score of 95.1! I think that explains why cows are not smarter.

This is not surprising, people with higher IQ have different educational pathways, different occupations, and are more interested in their health, all on the average and so might be more likely to be vegetartian. The fact that some of the "vegetarians" eat some chicken etc is irrelevant, the group is more vegetarian than the non-vegetarians.

What is more difficult is untangling the relationship between behaviours and outcomes such as coronary heart disease. Once people are interested in their health they will adopt a combination of allegedly healthy practices. People who use sunscreen have lower rates of heart disease, has been identified in one study, as well as various vitamin supplements. This type of study has many problems.